c语言实现SubString

#include<stdio.h>

#define N 50

int substring(char *, int, int, char *);

main()
{
int start,len,count;
char s[N];
char t[N];
printf("请输入字符串:\n");
gets(s);
printf("请输入起始位置:");
scanf("%d",&start);
printf("请输入所取字符长度:");
scanf("%d",&len);
count=substring(s,start,len,t);
if(count==-1)
   printf("起始值超出范围!\n");
else if(count==-2)
   printf("长度超出范围!\n");
else
{
   printf("得到的字符串是:");
   puts(t);
   getch();
}
return 0;
}

int substring(char * s,int start,int len,char * t)
{
int i=0,j=0;
while(s[i])
   i++;
if(i<start)
   return -1;
else if(i<start+len)
   return -2;
else
{
   for(i=start-1;i<=start+len-2;i++)
   {
    t[j]=s[i];
    j++;
   }
   t[j]='\0';
   return 1;
}
}